You insert your debit card. You pump the gas. You cringe (the current average price- per-gallon for premium is $3.62) and you leave the filling station assuming the $40 total will be subtracted from the balance in your debit card account. Simple, right?

What you don’t know is that behind the scenes, computers are doing their thing and when they are done, your bank may have put a pre-authorized hold on your account. It may be larger than your actual balance, especially if you are chronically hovering about the edges of the account. That means that when you drive from the gas station to the grocery store, your $15 purchase there may not be covered by the debit account.

For example, this made-up scenario: You have $100 in your account balance. You purchase $80 worth of merchandise or services (restaurants and hotels may also use this approach, as well as gas stations). The bank puts a pre-authorization hold on your account for $80. So the $20 you thought you had in the account is not available and additional use of the card will not be honored, which could mean additional fees.  Some credit card users have found themselves in serious trouble with overdrafts, not to mention the embarrassment and inconvenience involved.

In essence, your “available” balance and your “actual” balance can be two different figures.  The amount of the hold is deducted from your accounts “available” balance but not from the “actual” balance until the transaction that triggered the hold is processed, which could be several days.  Paying attention to your  “available” balance will help, but given the time frames involved, may not always provide the needed information as quickly as necessary.

“I don’t think the average customer has any idea what goes on behind the scenes when they swipe a card,” said Jeff Lenard, vice president of communications for the National Association of Convenience Stores (which sell about 80 percent of the gasoline purchased in the United States each year.)  “The credit and debit card system is incredibly complex.”

The problems associated with pre-authorized holds are only part of the potential trouble with using cards for gas station transactions.  Financial gurus also point out that the stations are a “danger zone” for theft of card data for fraudulent uses. Julie McNellely, senior analyst for Aite Group, a Boston-based financial services research organization, describes how bad guys bent on fraud use the stations to achieve their ends.

While customers are using their debit cards to make purchases at the pump, a thief sitting in a car nearby, armed with a laptop, a pinpoint camera and an antenna, has skimmed off the vital information from the card, including, if the camera is able to get a good view, the PIN. Before the unwary customer has even arrived at home, the information may have been put to fraudulent use.

Experts familiar with the potential for fraud at the gas station suggest the use of cash or credit cards, rather than debit cards, when filling up.